Suspensions of titanium (IV) oxide particles and process for their production

ABSTRACT

The disclosure pertains to a process for making a suspension of finely divided titanium (IV) oxide particles, comprising:
         vigorously mixing   (a) a volume of a first component comprising a major proportion of an alcohol, a minor proportion of a titanium alkoxide and a minor proportion of a titanium alkoxide activator selected from the group consisting of water and a first aqueous base, and   (b) a volume of a second component selected from the group consisting of water and a second aqueous base,   at least one of the first component or the second component having a base therein, the second component being substantially free of alcohol, to form a mixture comprising a suspension of finely divided titanium (IV) oxide particles, the mixture having a water to titanium molar ratio ranging from about 40 to about 1 to about 5000 to about 1,   wherein the proportion of the titanium alkoxide, the proportion of the activator, the mixing vigor, and a ratio of the volume of the first component to the volume of the second component are effective for the suspension of the finely divided particles to form in the mixture.       

     The suspensions can be dried to recover a powder. Typically the particles are nanoparticles.

RELATED APPLICATION

This application claims the benefit of Provisional Application No.60/876,218 filed on Dec. 21, 2006, incorporated herein by reference inits entirety.

FIELD OF THE DISCLOSURE

The disclosure relates to processes for making suspensions of finelydivided titanium (IV) oxide particles, in particular, suspensions oftitanium (IV) oxide nanoparticles.

BACKGROUND

The commercial utility of nanoparticles continues to grow at a rapidpace. Two examples of large and expanding markets for nanocrystallineand nanoparticulate titanium dioxide (also commonly called “titania”)are (1) optically transparent UV-absorbing coatings, i.e., coatings thatabsorb UV radiation but transmit visible light and (2) photocatalysisfor air and water purification and in antibacterial applications. Majorconsiderations for the titania/metal-oxide growth markets are to produceunique chemical and physical properties (such as highly reactivephotochemistry for catalysis, and high optical transparency with lowlevels of photoreactivity for coatings) in combination with competitivemanufacturing processes. The generation of new polymer/nanoparticlecomposites (“nanocomposites”) is an area with high potential that iscurrently the subject of intense investigation.

Large-scale commercial nanocrystalline TiO₂ can be made from titanylsulfate via acid-base chemistry, followed by calcination which provideshighly agglomerated particles. A nanoscale material can be produced viathe flame hydrolysis of TiCl₄; however, this material is also highlyagglomerated (typical agglomerate sizes are well over 300 nm). Fornumerous end-uses more finely divided particles are highly desirable.Since a significant amount of energy is required to reduce the size ofthe agglomerated particles, there has been a long felt need for aprocess which can synthesize finely divided titanium dioxide particleswithout the need for energy-intensive post-synthesis mechanical orchemical size reducing processes.

Titanium oxide particulates can be formed by hydrolyzing titaniumalkoxides with water or a base. Titanium alkoxides are known to reactrapidly with water to form a precipitate of agglomerated particles thatmust be milled to break-up the agglomerates to form small particles. JP[1989]-133939 describes a method for making titanium oxide particulatesby dissolving titanium alkoxide in alcohol with a water content of 3g/liter or less, and mixing the titanium-containing solution with analcohol containing water and ammonia with the resultant mixture havingspecific mole-ratio ranges of NH₃/Ti and H₂O/Ti. EP 0275688 and U.S.Pat. No. 4,861,572 describe a first titanium alkoxide hydrolyzation stepfollowed by a condensation reaction to form the metal oxide havingparticle sizes over 1 micron in size. JP 2001-246247 describes asubsequent acid or base treatment after hydrolysis of titanium alkoxideto produce relatively large particles of TiO₂.

For optically transparent cosmetic or coating applications, it isimportant that particles not inhibit the transmission of visible light,which requires most of the particles to be smaller than about 100 nm.

It has been found that without adequate dispersion and colloidalstabilization, nanoparticles tend to form aggregates that are difficultto re-disperse and that affect optical properties in the finalapplication. For some applications, it is preferable for thenanoparticles to remain in suspension, preferably a suspension resultingfrom the synthesis, in order to preserve their size and hence theirproperties.

The process of this disclosure can provide an optically transparentsuspension of titanium (IV) oxide nanoparticles.

SUMMARY OF THE DISCLOSURE

The disclosure relates to a process for making a suspension of finelydivided titanium (IV) oxide particles, typically, finely dividedtitanium (IV) oxide nanoparticles, comprising:

vigorously mixing

(a) a volume of a first component comprising a major proportion of analcohol, a minor proportion of a titanium alkoxide and a minorproportion of a titanium alkoxide activator selected from the groupconsisting of water and a first aqueous base, and

(b) a volume of a second component selected from the group consisting ofwater and a second aqueous base

at least one of the first component or the second component having abase therein, the second component being substantially free of alcohol,

to form a mixture comprising a suspension of finely divided titanium(IV) oxide particles, the mixture having a water to titanium molar ratioranging from about 40 to about 1 to about 5000 to about 1,

wherein the proportion of the titanium alkoxide, the proportion of theactivator, the mixing vigor, and a ratio of the volume of the firstcomponent to the volume of the second component are effective for thesuspension of the finely divided particles to form in the mixture.

The suspension can have a low opacity, additionally the suspension canbe translucent or optically transparent. When the suspension istranslucent, it can have an optical transparency greater than about 4%over a 1 cm pathlength based on the total amount of light transmittedthrough the suspension. When the suspension is transparent, it can havean optical transparency greater than about 90% over a 1 cm pathlength,based on the total amount of light transmitted through the suspension.

The base of the first component or the second component can be the sameor different and can be selected from the group consisting of ammoniumhydroxide, sodium hydroxide, sodium pyrophosphate, potassiumpyrophosphate, sodium citrate, diammonium hydrogen phosphate andmixtures thereof. In one embodiment, the base of the first component orthe second component or both is a weak base. The base can be atetraalkylammonium hydroxide in which the alkyl group contains from 1 to8 carbon atoms, typically, the base can be selected from the groupconsisting of tetraethylammonium hydroxide, tetra-n-butylammoniumhydroxide and mixtures thereof. When base is present in the firstcomponent the concentration of base can be up to about 0.2 M in thefirst component. When base is present in the second component theconcentration of base can range from 0.001 M to about 0.030 M in thesecond component. In one embodiment, the second component comprises abase and the base is selected from the group consisting oftetraethylammonium hydroxide and tetra-n-butylammonium hydroxide in aconcentration up to about 0.5 M. In one embodiment, the second componentcomprises a base which is ammonium hydroxide in a concentration rangingfrom about 0.006 M to about 14.8 M in the second component.

The amount of the titanium alkoxide can be up to and including about 20weight percent based on the entire weight of the first component whenthe aqueous base contains a tetraalkyl ammonium cation. Alternately;with a different base the amount of the titanium alkoxide can be nogreater than about 6 weight percent based on the entire weight of thefirst component.

The activator can range from about 0.2 to about 1.6 weight percent,typically about 0.8 wt. %, based on the entire weight of the firstcomponent.

The ratio of the volume of the first component to the volume of thesecond component can range from about 1 to about 1 to about 9 to about1, typically, the ratio of the volume of the first component to thevolume of the second component can be about 3 to about 1.

The titanium alkoxide can have the chemical structure:Ti(OR₁)(OR₂)(OR₃)(OR₄)wherein R₁ to R₄ are the same or different alkyl groups of 1 to about 30carbon atoms. Typically, the titanium alkoxide is selected from thegroup consisting of titanium (IV) isopropoxide, titanium (IV)n-butoxide, titanium (IV) methoxide, titanium (IV) ethoxide, andtitanium (IV) n-propoxide and mixtures thereof.

The alcohol of the first component is a monohydric or dihydric C₁ to C₁₀aliphatic alcohol. Typically, the alcohol is selected from the groupconsisting of methanol, ethanol, n-propanol, isopropanol, ethyleneglycol, and butanediol and mixtures thereof.

In one embodiment, the vigorous mixing is provided by a mixer with arotor and a stator and the rotational speed can be greater than about500 rpm. In this instance the shear rate is typically greater than about100 s⁻¹.

In one embodiment, the suspension of titanium (IV) oxide is dried toform a powder having the chemical composition:TiO_(2-x)(OH)_(2x) .n(H₂O)wherein x ranges from 0-2 and n is no greater than about 2, typicallyless than 2, more typically less than or equal to 1.

In another embodiment, the suspension of titanium (IV) oxide can beconcentrated by removing at least a portion of the liquid medium of thesuspension.

In yet another embodiment, the first component and the second componentcan be mixed in a continuous or a semi-continuous process.

In a still further embodiment, the suspension can be mixed with apolymer.

The disclosure provides a process for making titanium (IV) oxideparticles, preferably, nanoparticles.

The volume-weighted median particle size of a suspension of titanium(IV) oxide nanoparticles made in accordance with this disclosure,measured via dynamic light scattering, is below approximately 100 nm,and typically below about 10 nm. In many typical examples of the productof the disclosure the d₉₀ (90^(th) percentile of the volume-weightedcumulative particle size distribution) also is below 10 nm. Thesuspension is substantially free of a settled layer of agglomeratedsolids over time indicating that the suspension is stable. The particlesof the suspension are finely divided titanium (IV) oxide particles. Thefinely divided particles are typically substantially or fullyunagglomerated nanoparticles which is surprising since nanoparticlestend to form agglomerates.

The particles in the suspension can be substantially or completelyamorphous.

The disclosure provides a process which can synthesize finely dividedtitanium (IV) oxide particles in suspension without the need forenergy-intensive post-synthesis mechanical or chemical size reducingprocesses to size-reduce large agglomerates.

The process of the disclosure can provide, in as-synthesized form, adispersion of the particles which are colloidally stable.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 a and 1 b are graphs showing the particle size distribution ofhydrous titanium (IV) oxide suspensions as determined by dynamic lightscattering.

FIG. 2 depicts the UV-Visible absorption spectra for suspensions ofhydrous titanium (IV) oxide.

FIG. 3 compares the transmission spectra for suspensions of hydroustitanium (IV) oxide.

FIG. 4 shows theoretical and experimentally observed opticaltransparency values (measured across a 1 cm path length) as a functionof median particle size for a variety of suspensions of titanium (IV)oxide.

FIG. 5 shows the particle size distribution of a suspension made bydiluting the concentrated hydrous titanium (IV) oxide suspensionproduced in Example 29.

FIG. 6 is a simplified schematic diagram of the T-mixer.

DETAILED DESCRIPTION

Disclosed are processes for producing suspensions of particles oftitanium (IV) oxide and hydrous titanium (IV) oxide and compositions ofmatter that can be synthesized using titanium alkoxides as the startingmaterials. The term titanium (IV) oxide as used herein means titanium(IV) oxide and hydrous titanium (IV) oxide.

When separated from the suspension by drying under conditions suitablefor avoiding crystallization, the titanium (IV) oxide can have thechemical composition:TiO_(2-x)(OH)_(2x) .n(H₂O)

wherein x ranges from 0-2 and n is no greater than about 2, typicallyless than 2, more typically less than or equal to 1. The conditionssuitable for avoiding crystallization can be by drying in air at about20 to about 25° C.

The titanium (IV) oxide particles can be said to be directly synthesizedby contacting a first component and a second component in accordancewith the disclosure since there is no need for subsequent particleisolation steps such as vacuum distillation or size-reducing steps.Thus, in one embodiment, the particles can be made by a processconsisting essentially of contacting the first component and the secondcomponent.

The first component comprises a titanium alkoxide. The titanium alkoxidecan be represented by the chemical structure:Ti(OR₁)(OR₂)(OR₃)(OR₄)wherein R₁ to R₄ are the same or different alkyl groups of 1 to about 10carbon atoms, preferably 1 to about 6 carbon atoms and, optionally, oneor more heteroatoms such as oxygen, nitrogen or sulfur. The hydrocarbongroup can be a straight chain or branched and be saturated orunsaturated. The number of carbon atoms of the hydrocarbon can belimited by the solubility of the alkoxide in the alcohol. Specificexamples of suitable alkoxides include titanium (IV) isopropoxide(commercially available from E.I. du Pont de Nemours and Company underthe name Tyzor® TPT (Ti(OC₃H₇)₄) and titanium (IV) n-butoxide(commercially available from E.I. du Pont de Nemours and Company underthe name Tyzor® TBT (Ti(OC₄H₉)₄). Additional examples of titaniumalkoxides that can be useful are titanium (IV) methoxide, titanium (IV)ethoxide, and titanium (IV) n-propoxide. Mixtures of the foregoingtitanium alkoxides may be used.

The first component comprises an alcohol. Typically, the alcohol is amonohydric or dihydric C₁ to C₁₀ aliphatic alcohol. Suitable alcoholsinclude, but are not limited to, methanol, ethanol, n-propanol,isopropanol, ethylene glycol, and butanediol and mixtures thereof. Thealcohol is present in the first component in a major proportion based onthe total amount of the first component. Typically the alcohol ispresent in an amount of more than about 50 weight percent, moretypically more than about 80 weight percent, even more typically morethan about 90 weight percent based on the total weight of the firstcomponent.

The first component contains a titanium alkoxide activator, typically ina minor proportion, selected from the group consisting of water and atleast one aqueous base. The activator activates the titanium alkoxide.The source of water can be from the alcohol if the alcohol containswater of hydration in which water molecules are absorbed into thealcohol, examples of which include diols such as ethylene glycol.

The titanium alkoxide activator is present in a minor proportion basedon the entire amount of the first component. Typically the amount ofactivator can range from about 0.2 to about 1.6 wt. %, preferably about0.8 wt. % based on the entire weight of the first component.

The base of this disclosure suitable for use in the first aqueous basecan be organic or inorganic so long as it is water soluble. Materialswhich are solid at room temperature that are capable of providing analkaline pH when dissolved in water can be used. Suitable examples ofbases are selected from the group consisting of ammonium hydroxide,sodium hydroxide, sodium pyrophosphate (e.g., Na₄P₂O₇.10H₂O), potassiumpyrophosphate (e.g., K₄P₂O₇), sodium citrate (e.g., Na₃C₆H₅O₇.2H₂O),diammonium hydrogen phosphate (e.g., (NH₄)₂HPO₄) and mixtures thereof.Tetraalkylammonium hydroxides in which the alkyl group contains from 1to 8 carbon atoms can also be suitable as a base. Examples of suitabletetraalkylammonium hydroxides are selected from the group consisting oftetraethylammonium hydroxide (e.g., (CH₃CH₂)₄NOH), tetra-n-butylammoniumhydroxide (e.g., (CH₃CH₂CH₂CH₂)₄NOH) and mixtures thereof. The base ofthe first component, when present, is mixed with water to form a firstaqueous base.

When barium hydroxide was tested in the first component it was found tocause agglomeration under the specific conditions of the tests. Thus,barium hydroxide may not be a suitable base but suitable conditionswithin the scope of this disclosure may be found. As such, bariumhydroxide or mixtures of barium hydroxide with other bases describedherein are not to be excluded by the present disclosure.

When base is present in the first component the concentration generallycan be as high as about 0.2 M, typically about 0.1 M, more typicallyabout 0.001 to about 0.05 M in the first component.

The concentration of titanium alkoxide in the first component can varydepending upon the presence of base and the selection of type of base inthe first component. In any event the titanium alkoxide is present in aminor proportion based on the entire amount of the first component.Typically a minor proportion means that the titanium alkoxideconcentration in the first component can be as high as about 10 wt. % orhigher, typically upto and including about 20 wt. %, based on the entireweight of the first component. Usually, the titanium alkoxideconcentration does not exceed about 6 wt. %, typically about 5 wt. % andis preferably no more than 4 wt. % based on the entire weight of thefirst component. However, when tetra-alkylammonium hydroxide is presentin the first component the titanium alkoxide concentration can be about10 wt. % or higher such as up to and including about 20 wt. %. The lowerlimit of the titanium alkoxide concentration is greater than zero,typically the lower limit is about 0.001 wt. %, more typically about0.01 wt. %, even more typically about 0.02 wt. %.

The first component can be prepared by mixing the alcohol with theactivator. If sufficient water of hydration is present in the alcoholthe water of hydration can function as the activator. The mixture ofalcohol and the activator can then be mixed with the titanium alkoxide.Adding the titanium alkoxide after mixing the alcohol with the activatoris useful for avoiding forming a precipitate in the first component. Amechanism for rapidly dispersing the activator throughout the titaniumalkoxide-alcohol mixture could avoid precipitate formation. As anexample, spraying the activator into a mixture of alcohol and titaniumalkoxide while mixing could avoid forming a precipitate.

The first component can be prepared under ambient conditions, typicallyroom temperature, for example from about 20 to about 25° C. andatmospheric pressure. The temperature need not be elevated above ambienttemperatures.

The second component comprises water or at least one aqueous base. Thesecond component is substantially free of alcohol. The second componentbeing substantially free of alcohol is not meant to exclude traceamounts of alcohol which might be present as an impurity in theingredients such as when commodity materials are used. Alternately, thesecond component can be free of alcohol.

If a base is present in the first component, the base in the secondcomponent can be the same as or different from the base of the firstcomponent.

The base of this disclosure suitable for use in the second component canbe organic or inorganic so long as it is water soluble and can be thesame or different from the base of the first component, if a base ispresent in the first component. Materials which are solid at roomtemperature that are capable of providing an alkaline pH when dissolvedin water can be used. Suitable examples of bases are selected from thegroup consisting of ammonium hydroxide, sodium hydroxide, sodiumpyrophosphate (e.g., Na₄P₂O₇.10H₂O), potassium pyrophosphate (e.g.,K₄P₂O₇), sodium citrate (e.g., Na₃C₆H₅O₇.2H₂O), diammonium hydrogenphosphate (e.g., (NH₄)₂HPO₄) and mixtures thereof. Tetraalkylammoniumhydroxides in which the alkyl group contains from 1 to 8 carbon atomscan also be suitable as a base. Examples of suitable tetraalkylammoniumhydroxides are selected from the group consisting of tetraethylammoniumhydroxide (e.g., (CH₃CH₂)₄NOH), tetra-n-butylammonium hydroxide (e.g.,(CH₃CH₂CH₂CH₂)₄NOH) and mixtures thereof. The base of the secondcomponent, when present, is mixed with water to form a second aqueousbase.

The target pH of the second component can be above about 8, typicallyabout 9 or even higher, such as 11.

The concentration of base present in the second component can varydepending upon the choice of base and whether base is present in thefirst component. The concentration of base in the second component cantypically range from about 0.001 M to about 0.030 M, preferably about0.002 M to 0.016 M, and most preferably about 0.003 M to 0.010 M in thesecond component.

However, tetraethylammonium hydroxide and tetra-n-butylammoniumhydroxide can be used in effective concentrations higher than indicatedabove, up to about 0.5 M, more typically up to about 0.25 M even moretypically up to about 0.2 M, in the second component, to synthesizetitanium (IV) oxide nanoparticles in a suspension of about 3 wt. %titanium (IV) oxide based on the total amount of dry titanium (IV) oxidethat could be obtained from the suspension. The lower limit of thetetraethylammonium hydroxide and tetra-n-butylammonium hydroxideconcentration can be about 0.001 M, typically about 0.002 M, moretypically about 0.003 M, in the second component.

Moreover, ammonium hydroxide is a preferred base that can be used in thesecond component. The concentration of ammonium hydroxide, in the secondcomponent, can range from about 0.006 M to about 14.8 M, with about0.04-0.06 M ammonium hydroxide being preferred.

A base is present in at least one of the first and second components.Thus, when the first component comprises alcohol, water and titaniumalkoxide and is free of added base, the second component comprises anaqueous base, and when base is present in the first component the secondcomponent can be free of base.

When the second component comprises an aqueous base, it can be preparedby mixing the base with the water, typically deionized water. When thebase is a solid at room temperature, such as tetrasodiumpyrophosphatedecahydrate, a minor amount of the solid is dissolved in a major amountof water with stirring. When the base is a liquid at room temperature aminor amount can be added into a major amount of water while stirring.The second component can be prepared under ambient conditions, forexample about 20 to about 25° C. and atmospheric pressure.

The first component and the second component are contacted underconditions effective for forming a suspension of titanium (IV) oxideparticles which is substantially free of agglomerates. Additionally, themixture can be substantially optically transparent.

To avoid the formation of a slurry of agglomerated particles, a volumeof the first component can be added to a volume of the second componentand the ratio of the volume of the first component to the volume of thesecond component can range from about 1 to about 1 to about 9 to about1, preferably about 3 to about 1, i.e., 3 parts first component to 1part second component.

The second component can contain a significant stoichiometric excess ofwater. The total amount of water of the mixture of the first componentand the second component is significantly greater than required by thestoichiometry. The mixture of components can have molar ratios of waterto titanium, greater than about 40 to about 1. Typically, the totalwater to titanium molar ratio of the mixture ranges from about 40 toabout 1 to about 5000 to about 1, more typically from about 50 to about1 to about 600 to about 1, and even more typically from about 85 toabout 1 to about 300 to about 1.

Surprisingly, at such high water concentrations a suspension of finelydivided titanium (IV) oxide nanoparticles which are substantially freeof agglomerates and, additionally, having low opacity, more additionallysubstantially optically transparent, can be formed.

The target pH of the mixture of the components is typically greater thanor equal to about 6, more typically greater than about 7. By way ofexample only, when the first component is free of base, but containswater, and the second component contains ammonium hydroxide in aconcentration ranging from about 0.04 to about 0.06 M, the pH of theresulting suspension can be about 7 indicating consumption of hydroxideions. In the process of the disclosure the particles are formed in aneutral to basic pH range. Such pH values thereby avoid an environmentin which the particles tend to agglomerate.

Since the first component can have a short shelf-life, in one embodimentof the disclosure that can be useful for large scale production, amixture of alcohol and activator is established to flow, by gravity orby a pump, into a flow of the second component. The alkoxide is injectedinto a flow of the mixture of alcohol and activator before the flow ofthe mixture of alkoxide, alcohol and activator contacts the secondcomponent.

The first component and the second component are vigorously mixed. Byvigorously mixing it is meant that there is a substantial absence oflaminar flow when the components are mixed. Increasing the vigor ofmixing has been found to relate to increasing transparency of thesuspension The conditions for contacting the first component and thesecond component can be turbulent or can have a high rate of shear inorder to mix the first component and the second component underconditions that can provide a suspension which is substantially free ofagglomerates and substantially optically transparent. This contactingstep can be carried out with sufficient vigor to create a substantiallyoptically transparent suspension of titanium (IV) oxide nanoparticleswithin the time scale of the process. Typically the mixing shear rate isover about 100 s⁻¹, more typically the shear rate can range from about1000 to about 20,000 s⁻¹. Alternatively, the mixing vigor can beaccomplished by contacting the components using a static mixer or otherwell known mixing apparatus that can provide a high rate of mixing.

Mixing conditions preferably take into account how rapidly thecomponents are contacted while one of the components is agitated, forexample by stirring. It has been found that even when one component isstirred, dropwise addition of the other component does not form asuspension of finely divided particles.

Vigorous mixing can be terminated once a suspension of titanium (IV)oxide particles is formed. Mixing, even gentle mixing, is not alwaysrequired to maintain the suspension.

The first component and the second component can be contacted in anyorder. The second component can be added to the first component or thefirst component can be added to the second component or the first andsecond components can be mixed together simultaneously.

In one embodiment, the second component is poured into the firstcomponent which is in a stirred container. In a batch process, a vesselcan be provided which contains either the first or the second component.Typically the component of the vessel is agitated by rapid stirring witha mechanical stir bar. While the component is being agitated, the othercomponent is added to the vessel. The speed of addition can direct theoptical clarity of the resulting suspension. Slow addition can lead to atranslucent suspension within the scope of the disclosure which containsnanoparticles. Slow addition can mean that the other component is addedover a period of time ranging from about 5 seconds up to about oneminute. For high transparency, fast addition is preferred. Fast additioncan mean that the other component is added in about five seconds orless.

The titanium (IV) oxide can be made in a continuous or a semi-continuousprocess.

For small batch production, on the order of less than about 1000 mL, thesecond component can be rapidly poured by hand into the first component.The first component, typically, is of larger volume than the secondcomponent. The first component, typically, is in a stirred containerwherein the stir bar of the stirred container can be located off-centerto intensify stirring. However, any suitable technique for maintaininghighly turbulent conditions while contacting the first component and thesecond component can be employed especially for larger scale production.Examples of suitable equipment for maintaining highly turbulentconditions include a homogenizer wherein the rotor speed is greater thanabout 500 rpm, typically from about 1000 to about 30,000 rpm, and aT-mixer in which feedstreams of the first component and the secondcomponent are directed towards each other in a manner that can provide asufficient flow condition for sufficiently complete and, preferably,rapid mixing.

A simplified drawing of a T-mixer is shown in FIG. 6. The T-mixercomprises a mixing tube 2 having an upper portion 3 with two feed tubes4 a and 4 b fitted at the upper portion of the mixing tube 2 each feedtube having an end 6 a and 6 b positioned perpendicular to the mixingtube. Each feed tube is for introducing one of the components into themixing tube. The feed tubes are positioned within the mixing tube suchthat there is a gap 7 located between the ends of each feed tube. Thecross-sectional areas of the tubes can be chosen so that the volumetricflow rate required to mix the two components in the correct ratio wouldyield approximately equal velocities of the two jets.

The feed components are introduced to the mixing tube by way of the feedtubes so that they flow towards each other to form a mixture underturbulent mixing conditions.

Even when one of the components is rapidly agitated, for example, bystirring, fast addition of the other component is typically necessary toprovide high optical transparency. It has been found that when the firstand second components are contacted under conditions of low shear (i.e.,by pouring one into the other over the course of a few seconds) theparticles can agglomerate or even aggregate. In particular, it was foundthat drop-wise addition of the first component into the second componentcan produce large agglomerates with diameters of tens or hundreds ofmicrons. Such agglomerates and aggregates are difficult to disperse intonanoparticles.

The contacting conditions and the proportion of ingredients in the firstcomponent and the second component can yield a stable suspension offinely divided titanium (IV) oxide nanoparticles.

The d₉₀ (90^(th) percentile of the cumulative volume-weighted particlesize distribution), measured via dynamic light scattering, can be belowabout 100 nm, and typically below about 10 nm, typically having a lowerlimit of about 4 nm. Additionally, the particles can have avolume-weighted median size (d₅₀) of about 3 nm to about 20 nm, moretypically about 5 nm.

The suspensions in as-synthesized concentrations can be substantiallyoptically transparent. Thus, transparency values used herein refer tothe suspensions in the as-synthesized concentrations. Suspensions oftitanium (IV) oxide nanoparticles made following the process of thepresent disclosure can have transparencies greater than about 4%, andmore typically over about 90%. High optical transparency suspensions canbe made which are useful for end-uses requiring a high transparency suchas packaging film and clear containers. When high optical transparencysuspensions are desired they can be made such that the transparency(over a 1 cm path length) in the visible spectrum is greater than about80%, most preferably greater than about 95%, up to and including 100%,based on the total light transmission. Additionally opticallytranslucent suspensions can be made which permit the passage of light ina diffuse manner so that objects beyond cannot be clearly seen which canbe useful for end-uses that do not require high transparency.Translucent suspensions can have a transparency (over a 1 cm pathlength) in the visible spectrum of about 4% to about 20%.

The process of this disclosure avoids the need for energy-intensivepost-synthesis mechanical or chemical size-reducing processes such asgrinding in order to form finely divided titanium (IV) oxidenanoparticles.

The titanium (IV) oxide particles can have an intrinsically lowphotoactivity compared to crystalline titanium oxide, as describedherein below.

The ultraviolet light cut-off wavelength of the suspension can beapproximately 330 to approximately 350 nm, below which wavelengthultraviolet light is substantially blocked by the suspension.

As used herein, the term “suspension” means a mixture of titanium (IV)oxide particles that are more or less evenly distributed throughout aliquid medium, which is typically liquid at room temperature (from about20° C. to about 25° C.). The suspension is preferably substantially freeof flocs or agglomerates (i.e., clusters of particles held together byrelatively weak forces, such as van der Waals forces, electrostaticforces or surface tension). Even though the suspension may besubstantially free of flocs or, agglomerates, the incidental presence ofa very small number of flocs or agglomerates present as a minorconsequence of the process is not excluded. When the suspension is freeof flocs or agglomerates, however, it lacks even incidental flocs oragglomerates. The suspension is typically an easily flowable liquid atroom temperature (about 20° C. to about 25° C.) but it can be a gel atroom temperature.

The process of this disclosure can provide in situ stabilization of thetitanium (IV) oxide particles as they are formed which contributes tothe formation of finely dispersed particles and substantially opticallytransparent suspensions of titanium (IV) oxide nanoparticles.

In one embodiment, the mixture can be initially opaque and subsequentlyspontaneously form into a substantially optically transparentsuspension. In this embodiment, the process forms a composition whichcan provide in situ self-deagglomeration or“spontaneous peptization” ofthe titanium (IV) oxide particles under certain conditions. Thus, evenin cases where an opaque, white suspension is initially produced it hasbeen found that on standing, the suspension spontaneously transformsinto a substantially optically transparent suspension containing thefinely divided titanium (IV) oxide without additional processing suchas, grinding or other high energy input to convert the suspension ofagglomerated titanium (IV) oxide particles into a highly opticallytransparent suspension of titanium (IV) oxide nanoparticles evenlydistributed throughout the liquid medium.

With low volume ratios of the appropriate first component and secondcomponent, white opaque slurries can form. The opaque slurries mayinitially be formed having particles with median sizes of about 2microns. Typically the initially opaque slurry will have a transparencyof less than 5% over a 1 cm pathlength. The opaque slurries cantransform, or self-deagglomerate, into optically transparentsuspensions. Typically, the optically transparent suspensions containparticles having a median particle size of about 25 nm or less, andtypically as low as about 3 nm within about 1-180 minutes. This canoccur spontaneously on standing or with stirring. The low volume ratiosuitable for self-deagglomeration of the slurry can be from about 1 toabout 1 to about 3 to about 1, typically about 1 to about 1 to about 2to about 1. Thus, if spontaneous deagglomeration (i.e., spontaneouspeptization) is observed, then its time scale is the span of time afterthe two components are mixed together for the suspension to becomesubstantially optically transparent.

The particle size distribution (PSD) is affected by the presence of theactivator, relative concentrations of reactants, volume ratios of thefirst and the second components and the contacting conditions.

The first component and the second component can be contacted at atemperature ranging from just above the freezing point of the finalmixture up to the boiling point of the mixture. Typically, however, itis convenient to perform the reaction at temperatures below about 30°C., typically at ambient conditions, such as in the room temperaturerange of about 20 to about 25° C.

The present titanium (IV) oxide as-synthesized typically issubstantially amorphous but it is contemplated that crystalline materialcould be formed at higher reaction temperatures. The amorphous nature ofthe titanium (IV) oxide can provide the benefit of significantly reducedphotoactivity as high photoactivity tends to cause problems in certainapplications. However, if crystalline material is desired, it can bemade from the amorphous hydrous titanium (IV) oxide of the disclosure.The amorphous material can be converted to crystalline material,typically anatase, by heating to about 50° C. or higher, typically fromabout 80° C. to the boiling point at atmospheric pressure. Underpressure, higher temperatures can be used, as would be apparent to thoseskilled in the art. The material can be heated by boiling the suspensionfor an extended period of time. The period of time can depend on theingredients. The time periods can range from about 1 hour to about 72hours or even longer. Conversion to anatase crystals can be acceleratedby boiling the suspension in the presence of sodium pyrophosphate. Theheating process can be by induction heating.

The titanium (IV) oxide suspensions of the disclosure can compriseparticles with sizes well below 100 nm and can be highly opticallytransparent to visible light, fulfilling the optical requirements neededfor many end-uses. In addition, the suspension produced by thisdisclosure can exhibit ultraviolet (UV) absorption essentially the sameas that of nanocrystalline titanium (IV) oxide, making them potentiallysuitable for applications requiring such energy absorption (e.g., insunscreen cosmetics). Further, the suspensions, modified suspensions, orpowders derived from these suspensions, can be used in polymers and canproduce optically clear, tough, scratch-resistant coatings. Other usesfor the product or modified product of the disclosure may be envisaged,such as in chemical catalysis and photocatalysis for air and waterpurification or self-cleaning surfaces, or antibacterial and anti-odorapplications. Well-known methods for making end-use products containingtitanium (IV) oxide can be used.

High resolution Transmission Electron Microscopy (TEM) of titanium (IV)oxide particles produced at room temperature and atmospheric pressurefails to detect an electron diffraction pattern, suggesting that thetitanium (IV) oxide is amorphous, meaning that it has virtually nocrystalline structure.

A direct result of the lack of crystallinity is that the nanometrictitanium (IV) oxide described in the present disclosure is significantlymore photostable than either rutile or anatase, which are the commonlyoccurring forms of TiO₂ used in commerce. From a practical viewpoint,this finding means the amorphous particles do not require aphotopassivation treatment to prevent them from promoting harmfulfree-radical side reactions.

The suspensions produced by the process typically do not exceed atitanium (IV) oxide concentration of about 3 wt. % based on the entireweight of the suspension. The concentration can depend upon the choiceof base. Typically, concentrated suspensions (about 3 wt. % titanium(IV) oxide on a dry basis) of titanium (IV) oxide nanoparticles can bemade directly, without additional treatment of the sample, by usingtetra alkyl ammonium hydroxide such as tetra-n-butylammonium hydroxide(TnBAOH) and tetraethylammonium hydroxide (TEAOH).

However, for certain end uses requiring a higher concentration (over 3,typically over 5 wt. %) of titanium (IV) oxide nanoparticles insuspension the particle surface chemistry can be adjusted to minimizeparticle growth. The amorphous titanium (IV) oxide suspensions areunstable at lower pH, typically a pH below about 6 and becometranslucent gels. However, they are very stable (not forming a gel) at ahigher pH, typically above about 8, more typically a pH ranging fromabout 9 to about 10, even when a large excess of concentrated ammoniumhydroxide solution is used. Depending upon the starting concentration ofthe suspension, clear concentrates up to 17.8 wt. %, typically fromabout 0.1 wt. % to about 17.8 wt. %, based on the entire amount of thesuspension can be obtained by removing the liquid medium of thesuspension, for example by low-temperature (<40° C.) vacuum distillationof 0.5 wt. % suspensions. This method of concentrating the suspensionscauses very little agglomeration of the particles.

The particles produced by the process of this disclosure can be treatedwith a composition comprising an element selected from the transitionmetals, or Groups IIB or IVB of the Periodic Table of the Elements(Sargent-Welch Scientific Company, 1979). These treatments may beapplied using techniques known by those skilled in the art. Specificexamples of treatment may include inorganic oxide surface treatmentssuch as silica, alumina, zirconia among others. Such treatments may bepresent in an amount of 0.1 to 10 wt. %, based on the total weight ofthe particle, preferably 0.5 to 3 wt. %.

Any suitable silicon-, aluminum-, or zinc-containing compound which cansurface treat the particles with an oxide of silicon, aluminum or zinccan be used. Typically, the source of silicon is tetraethyl orthosilicate (TEOS), the source of aluminum is triethoxy aluminum and thesource of zinc is 2-methoxy ethoxide.

The particles can be surface treated by making a solution of the sourceof silicon, aluminum and/or zinc in dry alcohol such as dry isopropylalcohol. The solution can be introduced to a suspension of theparticles, typically by metering a quantity of the solution into thesuspension while stirring the suspension.

By “surface treated” it is meant that the particles have been contactedwith a source of the desired treatment wherein the compounds areadsorbed on the surface of the particles or, a reaction product of atleast one of the compounds with the particle is present on the surfaceas an adsorbed species or chemically bonded to the surface. Thecompounds or their reaction products or combination thereof may bepresent as a coating, either single layer or double layer, continuous ornon-continuous, on the surface. Typically, a continuous coatingcomprising the silicon-containing compound is on the surface. Thesurface treatment can reduce photoactivity.

The particles can be silanized as described in U.S. Pat. Nos. 5,889,090;5,607,994; 5,631,310; and 5,959,004, which are incorporated herein byreference in their entireties. As described in the foregoing patents,the particles may also contain ingredients added thereto to furtherimprove dispersibility characteristics of other properties such asdurability. Thus, by way of example, but not limited thereto, theparticles may contain additives and/or inorganic oxides, such asaluminum, silicon or tin as well as triethanolamine, trimethylolpropane,phosphates, phosphites, polyols and substituted polyols, silicones, andalkanolamines, such as triethanolamine.

The particles can be isolated by drying the suspension to form a powder.The suspension can be dried by spray drying. Spray drying can beaccomplished with an inert gas such as nitrogen. The powder can be afluidizable powder. The powder can be agglomerates of nanoparticleshaving a particle size distribution when the agglomerates are dispersedin water and measured wherein the volume-weighted particle size diameterof 50 percent of the particles (d₅₀) is less than about 100 nm,typically about 90 nm. The dry powders can be re-dispersed to provide asuspension having a larger particle size distribution than the particlesize distribution of the source suspension.

The suspensions of this disclosure can be synthesized easily and rapidlyat room temperature, or at any convenient temperature between thefreezing and boiling points of the solvent system selected. Producingdispersed titanium (IV) oxide particles in situ eliminates thehigh-energy grinding and milling procedures that would otherwise beneeded to disperse titanium (IV) oxide.

Concentrated suspensions made with this process can be incorporated intoan acrylic latex polymer to give a highly optically transparentcomposite film.

The products of this disclosure can be employed in purificationprocesses to remove contaminants, such as water purification, bywell-known techniques:

Description of Test Methods and Equipment

X-ray Powder Diffraction: Room-temperature powder x-ray diffraction datawere obtained with a Philips XPERT automated powder diffractometer,Model 3040. Samples were run in batch mode with a Model PW 1775 or ModelPW 3065 multi-position sample changer. The diffractometer was equippedwith an automatic variable slit, a xenon proportional counter, and agraphite monochromator. The radiation was CuK(alpha) (45 kV, 40 mA).Data were collected from 2 to 60 degrees 2-theta; a continuous scan withan equivalent step size of 0.03 deg; and a count time of 0.5 seconds perstep.

Particle Size Measurement: Volume-weighted particle size distributionswere measured on a Malvern Instruments Ltd. Zetasizer nano-S, which usesthe Dynamic Light Scattering (DLS) technique. The vendors software(version 4.10) was set to record 36 runs of 10 seconds each, with anequilibration time of 4 minutes at a temperature of 25 degrees Celsius.The“general purpose” (i.e., multi-modal) data inversion routine wasselected. Comparative Example C produced particles that were too largeto be measured with DLS, so the PSD of that sample was measured on aMalvern Instruments Mastersizer 2000, which uses the laser diffraction(also called “static light scattering”) technique.

As used in the examples, the median particle size refers to thevolume-weighted median size of the dispersed particles and not to thecrystallite size in polycrystalline material. It is possible forinstance to make a polycrystalline material with a high surface area andcrystallites less than 10 nm in size, yet that can only be partiallydispersed. The dispersed particles of such a sample would be aggregatestens or hundreds of microns in diameter. Since the crystallites in agiven aggregate particle would not be dispersed (separated from eachother), the size of the crystallites is to a first approximationirrelevant to the optical scattering. The optical transparency of thesuspension was determined by the particle size distribution of theaggregates.

The particle sizes reported throughout the specification refer to theparticle sizes measurements determined in accordance with this particlesize measurement procedure.

Optical Properties: One important characteristic of nanoparticles isthat, when properly dispersed, they are relatively optically transparentto visible light. Many commercial applications of nano TiO₂ require thatsome level of optical transparency be achieved without sacrificing otherimportant properties, such as protection against UV light. Therefore theultraviolet/visible (UV-Vis) optical properties of these hydroustitanium (IV) oxide suspensions were measured by absorption andtransmission spectroscopy.

Absorption Measurement: The absorption spectra were measured over a 0.1mm optical path. The spectra were obtained using a Cary 5 UV/VIS/NIRspectrophotometer. The spectrometer was controlled with a Dell PC andVarian WinUV version 3.0 software. A baseline was collected from 800 to200 nm with empty sample and reference beams. Sample solutions were thenloaded into a 0.1 mm quartz cell and placed in the sample beam. Thespectra were acquired versus the empty reference beam.

Transmission Measurement: The measurement of transmission, rather thanabsorption, is an alternative tool for examining differences betweenhighly optically transparent samples. Transmission spectra were measuredwith a spectrophotometer over a 1 cm optical path length defined by thedimension of the cuvette containing a sample. In order to account forreflections at the interfaces between air, cuvette, and suspension, acuvette containing only the continuous phase (e.g., a 70% alcohol/30%water mixture in the case of the suspensions produced by thisdisclosure) is measured first. These data are used to normalizesubsequent readings to obtain the transmission coefficient as a functionof wavelength.

For easy comparison of these results, the optical transparency factor (%T) is defined as the weighted average of the transmission coefficient,using the CIE photopic luminous efficiency function (PLEF) as theweighting factor. Thus the optical transparency is calculated as

${\%\mspace{14mu} T} = \frac{\sum\limits_{\lambda}\;{{{t(\lambda)} \cdot P}\; L\; E\;{F(\lambda)}}}{\sum\limits_{\lambda}{P\; L\; E\;{F(\lambda)}}}$

where t(λ) is the transmission coefficient (in percent) at eachwavelength λ, measured over a specified optical path length. For all thetransmission coefficient data shown in this disclosure, the optical pathlength is 1 cm.

UV-Visible spectra and determination of transparency were measured on anOcean Optics Inc. fiber-optic spectrophotometer Model S2000-UV-VIS,using an Analytical Instrument Systems Inc. Model Mini-DT deuteriumlight source and an Ocean Optics Model CUV cuvette holder. The spectrawere recorded with an integration time on the order of 30 ms, and theoptical path length through the sample was 1 cm.

Photoactivity Measurement: The photoactivity of the products of theExamples was determined by the phenol photooxidation test. Experimentswere performed with UV light from a 450 W Hg-vapor lamp in aphotochemical reactor immersion well. The test sample and a commercialDegussa P-25 titanium dioxide (ca. 75:15 anatase:rutile) control inindividual test tubes were symmetrically arranged in a carousel-likeapparatus in a temperature-controlled water bath. To facilitate thereaction, both O₂ sparging and magnetic stirring of each sample wasutilized to provide uniform mixing. To permit only UV radiation withindesired 300-400 nm range to reach the reaction solution (1 mM phenol), aPyrex® filter was put in place. The jacketed reactor was water-cooledthroughout the operation and for at least 6 min. after shutdown toprevent overheating of the lamp, thereby assuring proper operation. Inaddition to initial samples taken prior to UV exposure, lamp operationwas suspended during set intervals (5, 20, and 60 min.) to draw samplesfor gas chromatograph injections. The gas chromatography indicated thedisappearance of phenol due to photo-oxidation. A calibration curve ofthe neat phenol solution at different concentrations (1.0, 0.5, 0.2, and0.1 mM) was also measured, analyzed, and recorded for calibration ofeach experimental run. Relative reaction rate constants, based on theinternal P-25 standard, were calculated.

Shear Rate: The shear rate reported in the examples, measured in s⁻¹,was calculated by multiplying the rotational velocity (revolutions persecond) by the ratio of the rotor radius to the distance between therotor and the stator.

EXAMPLES

Except as otherwise noted, all chemicals and reagents of the Exampleswere used as received from

Methanol—EMD Chemicals, Gibbstown, N.J., 99.8%

Ethanol—EMD Chemicals, Gibbstown, N.J., anhydrous, denatured.

n-Propanol—EMD Chemicals, Gibbstown, N.J., 99.97%

Isopropanol—EMD Chemicals, Gibbstown, N.J., 99.5%

Ammonium hydroxide (NH₄OH)— J. T. Baker, Phillipsburg, N.J.,

Tetra sodium pyrophosphate (Na₄P₂O₇.10H₂O)— EMD Chemicals, Gibbstown,N.J., 99.0-103.0%

Tetra potassium pyrophosphate (K₄P₂O₇)— Aldrich Chemical Co., Milwaukee,Wis., 97%

Sodium citrate (Na₃C₆H₅O₇.2H₂O)—Sigma Chemical, St Louis, Mo.

Sodium hydroxide (NaOH)-EMD Chemicals, Gibbstown, N.J., 97.0% min.

Barium hydroxide (Ba(OH)₂.8H₂O), Alfa Aesar, Ward Hill, Mass., 98+%

Diammonium hydrogen phosphate ((NH₄)₂HPO₄)— J. T. Baker, Phillipsburg,N.J., 99.7.

Tetraethylammonium hydroxide ((CH₃CH₂)₄NOH)—Sigma Chemical, St. Louis,Mo., 25 wt. % in methanol.

Tetra-n-butylammonium hydroxide ((CH₃CH₂CH₂CH₂)₄NOH)— Alfa Aesar, WardHill, Mass., Reagent Grade, 31 wt. % in methanol.

Titanium (IV) isopropoxide—Aldrich Chemical Co., Milwaukee, Wis.,99.999%

Titanium (IV) isopropoxide—Aldrich Chemical Co., Milwaukee, Wis., 97%

Titanium (IV) n-butoxide—Aldrich Chemical Co., Milwaukee, Wis., 97%

The following Examples illustrate the present disclosure. All parts,percentages and proportions are by weight unless otherwise indicated.

Except as otherwise noted, the term “rapidly added” is used to mean thatthe entire contents of the beaker containing one of the components wasdumped, by hand, into the beaker containing the other component asquickly as possible (in less than about one second), and vigorous mixingwas provided by a stir bar positioned off the central axis of the beakerto increase the turbulence of the mixture to thoroughly mix thecomponents in less than about five seconds of total elapsed time. Theseconditions for rapidly mixing the components were effective for smallscale batchwise production of a suspension of amorphous hydrous titanium(IV) oxide.

Comparative Example A

This example illustrates the reaction of titanium (IV) isopropoxide(a.k.a, Tetraisopropyltitanate, Ti(OCH(CH₃)₂)₄) in isopropyl alcoholwith a base. No activator was present in Component “A”.

Component “A” was prepared by mixing 6 mL titanium (IV) isopropoxidewith 300 mL isopropyl alcohol. Component “B” was made by dissolving 0.36g tetrasodiumpyrophosphate decahydrate (TSPP) in 100 mL deionized water.Component “A” was stirred in a 600 mL beaker with a Teflon-coatedmagnetic stirring bar as Component “B” was rapidly added to Component“A”. A white slurry formed immediately indicating the formation of largeparticles. Because the precipitate was white, the solids were obviouslylarge enough (on the order of 200-300 nm) to scatter light effectively.Subsequent particle size measurements using dynamic light scatteringindicated a median particle size of over 300 nm.

Comparative Example B

This example illustrates the result of reaction of titanium (IV)isopropoxide (Tetraisopropyltitanate, Ti(OCH(CH₃)₂)₄) in isopropylalcohol containing about 0.8 wt. % H₂O with barium hydroxide as thebase.

With stirring from a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 150 mL isopropyl alcohol with 1 mL deionized waterfollowed by the addition of 3 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.1 g Ba(OH)₂.8H₂O in 50 mL deionized water.The pH of Component “B” was about 11. Component “A” was stirred in a 400mL beaker with a Teflon-coated magnetic stirring bar as Component “B”was rapidly added to Component “A”. A white opaque slurry of hydroustitanium oxide nanoparticles was formed.

Comparative Example C

In this Example the intermediate product of Example 1 of JP 2001-246247was repeated. Following the first part of Example 1 of JP 2001-246247,25 mL of isopropanol was added drop-wise to 25 mL of titaniumisopropoxide (Ti(OC₃H₇)₄); the mixture was stirred at 400 rpm at roomtemperature for 10 minutes to obtain Component “A”. The shear rate forthis geometry and stirring rate was calculated to be 16 s⁻¹. Next 100 mLof de-ionized water was mixed with 200 mL of isopropanol to obtainComponent “B”. Component “B” was added to the beaker containingComponent “A” while stirring at 400 rpm; the mixture was heated to 60°C. and stirred at 400 rpm for 2 hours. The slurry (calculated to beabout 2.2 wt. % titanium (IV) oxide on a dry basis) was opaque white,with a % T estimated to be less than 0.01%.

The measured transmission was reported in FIG. 3. The opticaltransparency was calculated and reported in Tables 1 and 2.

The median particle size was measured to be 54 microns (54,000 nm) usingconventional laser diffraction. This result is denoted as a filledsquare in FIG. 4.

The product of Comparative Example C was observed to be so opaque thatthe measured transmission was essentially zero at visible wavelengths.The product of Comparative Example C was diluted from about 2.2 wt. %titanium (IV) oxide to about 0.1 wt. % titanium (IV) oxide by adding onepart product of Comparative Example C to 21 parts of a 1:1 mixture ofalcohol and water mixture (chosen to mimic the composition of thecontinuous phase). The transmission spectrum for this diluted product ofComparative Example C is shown in FIG. 3 as a dashed line tagged with an“x” and running along the bottom of the graph.

Comparative Example D

This example illustrates that dropwise addition of a base to a solutionof titanium (IV) isopropoxide (Tetraisopropyltitanate, Ti(OCH(CH₃)₂)₄)in isopropyl alcohol containing about 0.8 wt. % H₂O does not give asuspension of the small particles of hydrous titanium oxide.

With stirring from a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 150 mL isopropyl alcohol with 0.5 mL deionized waterfollowed by the addition of 3 mL titanium (IV) isopropoxide. Component“B” was made by mixing four drops (˜0.12 mL) concentrated NH₄OH with 50mL deionized water. Component “A” was stirred in a 400 mL beaker with aTeflon-coated magnetic stirring bar as Component “B” was added in adropwise manner to Component “A”. A white opaque slurry formed afteronly a few mL of Component “B” were added. After the complete additionof Component “B”, and after stirring of the slurry was stopped, thewhite solid began to settle.

Examples 1-3 demonstrate synthesis of the substantially opticallytransparent suspension when the alcohol was believed to contain water ofhydration because the alcohols had been in storage over a long period oftime. The exact water concentration of the alcohol was unknown.

Example 1

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL ethyl alcohol believed to contain an amount ofwater of hydration with 6 mL titanium (IV) isopropoxide. Component “B”was made by dissolving 0.36 g tetrasodiumpyrophosphate decahydrate(TSPP) in 100 mL deionized water. Component “A” was stirred in a 600 mLbeaker with a Teflon-coated magnetic stirring bar as Component “B” wasrapidly added to Component “A”. A partially optically transparent (basedon visual appearance, estimated to fall within the range of 5%<% T<20%)suspension of hydrous titanium (IV) oxide nanoparticles was formed. On adry TiO₂ basis, the suspension contained about 0.5 wt. % TiO₂.

Example 2

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL n-propyl alcohol believed to contain an amountof water of hydration with 6 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.36 g tetrasodiumpyrophosphate decahydrate(TSPP) in 100 mL deionized water. Component “A” was stirred in a 600 mLbeaker with a Teflon-coated magnetic stirring bar as Component “B” wasrapidly added to Component “A”. A partially optically transparent (basedon visual appearance, estimated to fall within the range of 5%<% T<20%)suspension of hydrous titanium (IV) oxide nanoparticles was formed. On adry TiO₂ basis, the suspension contained about 0.5 wt. % TiO₂.

Example 3

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL methyl alcohol believed to contain an amountof water of hydration with 6 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.36 g tetrasodiumpyrophosphate decahydrate(TSPP) in 100 mL deionized water. Component “A” was stirred in a 600 mLbeaker with a Teflon-coated magnetic stirring bar as Component “B” wasrapidly added to Component “A”. A clear suspension (based on visualappearance, estimated to have a % T>80%) of hydrous titanium (IV) oxidenanoparticles was formed. On a dry TiO₂ basis, the suspension containedabout 0.5 wt. % TiO₂.

In the following Examples alcohol which was not believed to have beencontaminated with water because of long term storage was used along witha measured amount of water to produce substantially opticallytransparent suspensions.

Example 4

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL isopropyl alcohol with 0.5 mL deionized waterfollowed by the addition of 6 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.36 g tetrasodiumpyrophosphate decahydrate(TSPP) in 100 mL deionized water. Component “A” was stirred in a 600 mLbeaker with a Teflon-coated magnetic stirring bar as Component “B” wasrapidly added to Component “A”. A highly optically transparentsuspension of hydrous titanium (IV) oxide nanoparticles, resembling aclear colorless solution, was formed. The molar ratio of H₂O:Ti in thesuspension was about 279 to about 1. On a dry TiO₂ basis, the suspensioncontained about 0.5 wt. % TiO₂. The particle size distribution wasmeasured directly on this suspension by dynamic light scattering and themedian particle size was 6.9 nm; the particle size distribution isdepicted by the filled circles in FIG. 1 a.

Example 5

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 600 mL isopropyl alcohol with 2.0 mL deionized waterfollowed by the addition of 12 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.72 g tetrasodiumpyrophosphate decahydrate(TSPP) in 200 mL deionized water. Component “A” was stirred in a 1 Lbeaker with a Teflon-coated magnetic stirring bar as Component “B” wasrapidly added to Component “A”. A highly optically transparentsuspension of hydrous titanium (IV) oxide nanoparticles, resembling aclear colorless solution, was formed. The molar ratio of H₂O:Ti in thesuspension was about 280 to about 1. On a dry TiO₂ basis, the suspensioncontained about 0.5 wt. % TiO₂.

Example 6

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL isopropyl alcohol with 2.0 mL deionized waterfollowed by the addition of 6 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.27 g tetrapotassiumpyrophosphate (TKPP) in100 mL deionized water. Component “A” was stirred in a 600 mL beakerwith a Teflon-coated magnetic stirring bar as Component “B” was rapidlyadded to Component “A”. A highly optically transparent suspension ofhydrous titanium (IV) oxide nanoparticles, resembling a clear colorlesssolution, was formed. The molar ratio of H₂O:Ti in the suspension wasabout 283 to about 1. On a dry TiO₂ basis, the suspension containedabout 0.5 wt. % TiO₂.

Example 7

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL isopropyl alcohol with 2.0 mL deionized waterfollowed by the addition of 6 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.24 g sodium citrate (trisodium) dihydratein 100 mL deionized water. Component “A” was stirred in a 600 mL beakerwith a Teflon-coated magnetic stirring bar as Component “B” was rapidlyadded to Component “A”. A highly optically transparent suspension ofhydrous titanium (IV) oxide nanoparticles, resembling a clear colorlesssolution, was formed. The molar ratio of H₂O:Ti in the suspension wasabout 283, to about 1. On a dry TiO₂ basis, the suspension containedabout 0.5 wt. % TiO₂. The particle size distribution (depicted by theopen triangles in FIG. 1 a) was measured directly on this suspension bydynamic light scattering and the median particle size was 7.6 nm. Thetransmission spectrum for the product of this Example is denoted in FIG.3 by the line tagged with a filled circle; the optical transparency wasmeasured to be 95% and reported in Table 2.

Example 8

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL isopropyl alcohol with 2.0 mL deionized waterfollowed by the addition of 6 mL titanium (IV) isopropoxide. Component“B” was made by adding 8 drops (˜0.25 mL) concentrated ammoniumhydroxide (NH₄OH) to 100 mL deionized water with stirring. The pH ofComponent “B” was ˜10. Component “B” was rapidly added to stirringComponent “A”. A highly optically transparent suspension of hydroustitanium (IV) oxide nanoparticles, resembling a clear colorlesssolution, was formed. The molar ratio of H₂O:Ti in the suspension wasabout 284 to about 1. On a dry TiO₂ basis, the suspension containedabout 0.5 wt. % TiO₂. The pH of the nanoparticle suspension was ˜7. Theparticle size distribution was measured directly on this suspension bydynamic light scattering and the median particle size was 4.6 nm asdepicted by the filled triangles in FIG. 1 a. The measured opticaltransparency was about 97%.

Example 9

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL isopropyl alcohol with 2.0 mL deionized waterfollowed by the addition of 6 mL titanium (IV) isopropoxide. Component“B” was made by adding 1 drop (˜0.03 mL) concentrated ammonium hydroxide(NH₄OH) to 100 mL deionized water while stirring. The pH of Component“B” was ˜9. Component “B” was rapidly added to stirring Component “A”. Apartially optically transparent (based on visual appearance, estimatedto fall within the range of 5%<% T<20%, i.e. translucent) suspension ofhydrous titanium (IV) oxide nanoparticles was formed. On a dry TiO₂basis, the suspension contained about 0.5 wt. % TiO₂. The spectrum forthe product of this Example is depicted in FIG. 3 by the line taggedwith an open square; the optical transparency was measured to be 4% asshown in Table 2.

Example 10

The experiment was repeated with Component “A” prepared as describedabove in Example 9 and Component “B” prepared by adding 2 drops (˜0.06mL) concentrated ammonium hydroxide (NH₄OH) to 100 mL deionized waterwhile stirring. Component “B” was rapidly added to stirring Component“A”, and a more optically transparent suspension of hydrous titanium(IV) oxide nanoparticles was formed. The spectrum for this Example 10 isdepicted in FIG. 3 as a line tagged with an open triangle; the opticaltransparency was 33% as shown in Table 2.

Example 11

This Example 11 illustrates the addition of base to the alcohol-watersolution of Component “A”, and the use of water as Component “B”.

With stirring, Component “A” was prepared by mixing 150 mL n-propylalcohol, 1.0 mL deionized water, six drops (˜0.2 mL) concentrated NH₄OH,followed by the addition of 6 mL titanium (IV) isopropoxide. To thisstirring solution was rapidly added 45 mL deionized H₂O as Component“B”. A partially optically transparent (based on visual appearance,estimated to fall within the range of 5%<% T<20%) suspension of hydroustitanium (IV) oxide nanoparticles was formed. The molar ratio of H₂O:Tiin the suspension was about 128 to about 1. On a dry TiO₂ basis, thesuspension contained about 0.9 wt. % TiO₂.

Example 12

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL isopropyl alcohol with 2.0 mL deionized waterfollowed by the addition of 6.7 mL titanium (IV) isopropoxide. Component“B” was made by dissolving 0.4 g diammonium hydrogen phosphate((NH₄)₂HPO₄) in 100 mL deionized water. Component “A” was stirred in a600 mL beaker with a Teflon-coated magnetic stirring bar as Component“B” was rapidly added to Component “A”. A highly optically transparentsuspension of hydrous titanium (IV) oxide nanoparticles, resembling aclear colorless solution, was formed. The molar ratio of H₂O:Ti in thesuspension was about 252 to about 1. On a dry TiO₂ basis, the suspensioncontained about 0.5 wt. % TiO₂.

Example 13

This example illustrates the reaction of titanium (IV) isopropoxide inethylene glycol containing water of hydration to form Component “A”mixed with a base as Component “B”.

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 50 mL ethylene glycol containing water of hydrationwith 1 mL titanium (IV) isopropoxide. Component “B” was made by mixingtwo drops (about 0.07 mL) concentrated NH₄OH with 15 mL deionized water.Component “A” was stirred in a 150 mL beaker with a Teflon-coatedmagnetic stirring bar as Component “B” was rapidly added to Component“A”. A highly optically transparent suspension of hydrous titanium (IV)oxide nanoparticles, resembling a clear colorless solution, was formed.The molar ratio of H₂O:Ti in the suspension was about 245 to about 1. Ona dry TiO₂ basis, the suspension contained about 0.4 wt. % TiO₂.

Example 14

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 300 mL isopropyl alcohol with 2.0 mL deionized waterfollowed by the addition of 6.9 mL titanium (IV) n-butoxide. Component“B” was made by adding 8 drops (˜0.25 mL) concentrated ammoniumhydroxide (NH₄OH) to 100 mL deionized water with stirring. Component “A”was stirred in a 600 mL beaker with a Teflon-coated magnetic stirringbar as Component “B” was rapidly added to Component “A”. A highlyoptically transparent suspension of hydrous titanium (IV) oxidenanoparticles, resembling a clear colorless solution, was formed. Themolar ratio of H₂O:Ti in the suspension was about 283 to about 1. On adry TiO₂ basis, the suspension contained about 0.5 wt. % TiO₂. Theparticle size distribution was measured by dynamic light scattering andthe median size (d₅₀) was 6.7 nm as depicted by the open circles in FIG.1 a. The measured light transmission is reported in FIG. 3 as an opentriangle.

Example 15

Example 14 was repeated using 8.0 mL titanium (IV) n-butoxide withsimilar results. A highly optically transparent suspension of hydroustitanium (IV) oxide nanoparticles, resembling a clear colorlesssolution, was formed. The molar ratio of H₂O:Ti in the suspension wasabout 241 to about 1. On a dry TiO₂ basis, the suspension containedabout 0.5 wt. % TiO₂.

Example 16

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 150 mL isopropyl alcohol with 1.0 mL deionized H₂Ofollowed by the addition of 3 mL titanium (IV) isopropoxide. Component“B” consisted of 50 mL concentrated ammonium hydroxide. With allingredients at room temperature, Component “A” was stirred in a 400 mLbeaker with a Teflon-coated magnetic stirring bar as Component “B” wasrapidly added to Component “A”. A highly optically transparent fluidsuspension of hydrous titanium (IV) oxide particles was formedresembling a clear colorless solution that was stable for about twominutes before gelling. On a dry TiO₂ basis, the suspension containedabout 0.5 wt. % TiO₂.

Example 17

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 150 mL isopropyl alcohol with 1.0 mL concentratedNH₄OH followed by the addition of 3 mL titanium (IV) isopropoxide.Component “B” consisted of 50 mL deionized H₂O. Component “A” wasstirred in a 400 mL beaker with a Teflon-coated magnetic stirring bar asthe 50 mL of H₂O were rapidly added to Component “A”. A highly opticallytransparent suspension of hydrous titanium (IV) oxide nanoparticles,resembling a clear colorless solution, was formed. On a dry TiO₂ basis,the suspension contained about 0.5 wt. TiO₂. The pH of the suspensionwas about 10 as measured by multi-strip pH paper.

Example 18

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by heating to 75° C. a solution consisting of 150 mL isopropylalcohol and 1.0 mL deionized water in a 400 mL beaker. When 75° C. wasobtained, 3.0 mL titanium (IV) isopropoxide were added. Component “B”was made by heating 50 mL H₂O to 75° C., at which point, 6 drops (˜0.18mL) of concentrated NH₄OH were added with stirring. Component “A” wasstirred rapidly as Component “B” was rapidly added to Component “A”. Ahighly optically transparent suspension of hydrous titanium (IV) oxidenanoparticles, resembling a clear colorless solution, was formed. On adry TiO₂ basis, the suspension contained about 0.5 wt. % TiO₂. Theparticle size distribution was measured by dynamic light scattering andthe median particle size was 6.4 nm as depicted by the filled circles inFIG. 1 b. A sample of the product of this Example was aged for twomonths by placing the sample in a capped bottle and storing it at roomtemperature on a laboratory counter and the measured opticaltransparency of the aged sample was 66% as reported in Table 2. Thetransmission spectrum for the aged product of this Example is denoted inFIG. 3 by the line tagged with an open circle.

Examples 19-21 show spontaneous peptization of the products.

Example 19

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 100 mL isopropyl alcohol with 1.0 mL deionized waterfollowed by the addition of 3.0 mL titanium (IV) isopropoxide. Component“B” was made by mixing 6 drops concentrated NH₄OH (˜0.18 mL) in 50 mLdeionized water. Component “A” was stirred in a 400 mL beaker with aTeflon-coated magnetic stirring bar as Component “B” was rapidly addedto Component “A”. A cloudy white mixture was formed that became a highlyoptically transparent suspension, with the appearance of a clearcolorless solution, after about 10 minutes. On a dry TiO₂ basis, thesuspension contained about 0.6 wt. % TiO₂. The particle sizedistribution was measured by dynamic light scattering and the d₅₀ was2.4 nm as depicted by the open triangles in FIG. 1 b.

Example 20

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 100 mL isopropyl alcohol with 0.5 mL deionized waterfollowed by the addition of 3.0 mL titanium (IV) isopropoxide. Component“B” was made by mixing 6 drops concentrated NH₄OH (˜0.18 mL) in 50 mLdeionized water. Component “A” was stirred in a 400 mL beaker with aTeflon-coated magnetic stirring bar as Component “B” was rapidly addedto Component “A”. A cloudy white mixture was formed that clarified withtime, becoming a highly optically transparent suspension, with theappearance of a clear colorless solution, after about 10 minutes. On adry TiO₂ basis, the suspension contained about 0.6 wt. % TiO₂. Theparticle size distribution was measured by dynamic light scattering andthe median particle size was 2.2 nm as depicted by the filled trianglesin FIG. 1 b.

Example 21

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 100 mL isopropyl alcohol with 1.0 mL deionized waterfollowed by the addition of 3.0 mL titanium (IV) isopropoxide. Component“B” was made by mixing 12 drops concentrated NH₄OH (˜0.38 mL) in 100 mLdeionized water. Component “A” was stirred in a 400 mL beaker with aTeflon-coated magnetic stirring bar as Component “B” was rapidly addedto Component “A”. A cloudy white mixture was formed that slowlyclarified with time, becoming a highly optically transparent suspension,with the appearance of a clear colorless solution, after about 2 hours.The molar ratio of H₂O:Ti in the suspension was about 561 to about 1. Ona dry TiO₂ basis, the suspension contained about 0.4 wt. % TiO₂. Theparticle size distribution was measured by dynamic light scattering andthe median particle size was 4.1 nm as depicted by the open-circles inFIG. 1 b.

The following Examples 22-23 show the impact of the bases on the abilityto crystallize the amorphous titanium (IV) oxide particle via a heattreatment.

Example 22

This example illustrates that hydrous titanium (IV) oxide nanoparticlesmade by solution reaction of titanium (IV) isopropoxide with ammoniumhydroxide remain amorphous to X-rays even after boiling of thesuspension for 6 hours.

A suspension of hydrous titanium (IV) oxide nanoparticles was preparedas described in Example 8. A 50 mL portion of suspension was evaporatedto dryness at room temperature. An X-ray powder diffraction patternshowed the white powder to be amorphous.

Three 800 mL suspensions of hydrous titanium (IV) oxide nanoparticles,containing about 0.5 wt. % TiO₂ on a dry basis, were prepared in amanner similar to that described in Example 8. The three suspensionswere heated to boiling for 5 minutes, 1 hour, and 6 hours, respectively.All three suspensions were highly optically transparent after heatingwith only a very small amount of flocculated solid present, and thesuspensions resembled a clear colorless solution. Portions of heatedsuspension were evaporated to dryness at room temperature. X-ray powderdiffraction patterns showed all three powders to be amorphous.

Example 23

This example illustrates that hydrous titanium (IV) oxide nanoparticlesmade by solution reaction of titanium (IV) isopropoxide withtetrasodiumpyrophosphate as the base are converted to crystallineanatase after boiling of the suspension for 6 hours.

A suspension of hydrous titanium (IV) oxide nanoparticles was preparedat room temperature following Example 5, except that 4 mL of water(instead of 2 mL) was used for Component “A” and 0.18 g of TSPP (insteadof 0.72 g) was used for Component “B” A portion of the highly opticallytransparent suspension was evaporated to dryness at room temperature.X-ray powder diffraction of the recovered powder showed it to beamorphous.

Another batch of suspension of hydrous titanium (IV) oxidenanoparticles, containing about 0.5 wt. % TiO₂ on a dry basis, wasprepared in the same manner (i.e., using 2 mL of water and 0.18 g ofTSPP). The suspension was heated to boiling and refluxed for 6 hours. Aportion of heated suspension was evaporated to dryness at roomtemperature. An X ray powder diffraction pattern of the recovered powdershowed broad lines of anatase, and from the width of the strongest peak,an average crystal size of 6 nm was estimated.

Example 23 shows that the amorphous titanium (IV) oxide nanoparticlescan be converted to the anatase form through a heat treatment. Thecrystallinity was confirmed through X-ray diffraction analysis. Thecrystallization of the amorphous titanium (IV) oxide, and the resultingphotoactivity can, however, be controlled through judicious selection ofthe starting base. NH₄OH is expected to yield a relativelylow-photoactivity product in the phenol photo-oxidation test even afterboiling of the suspension for 6 hours (as described in Example 22)because the product of Example 22 was amorphous, whereas, under the sameheat treating conditions, tetrasodiumpyrophosphate is expected toproduce, in the phenol photo-oxidation test, a highly photoactivematerial because of the crystallinity of the product of Example 23.

Example 24

This example illustrates that suspensions of hydrous titanium (IV) oxidenanoparticles can be concentrated to about 3-4 wt. % TiO₂ on a drybasis.

A 1200 mL suspension of hydrous titanium (IV) oxide nanoparticles wasprepared by scaling up Example 8 by a factor of three. On a dry TiO₂basis, the suspension contained about 0.5 wt. % TiO₂. 12 mL concentratedNH₄OH were added to the suspension, and the suspension was vacuumdistilled at about 25° C. until it became viscous and cloudy with afinal volume of about 150 mL. On a dry TiO₂ basis, the concentratedsuspension contained about 3-4 wt. % TiO₂. A portion of the concentratedsuspension was evaporated to dryness at room temperature. X-ray powderdiffraction of the recovered powder showed it to be amorphous.

Example 25

This example illustrates that spray-dried powder obtained from asuspension of hydrous titanium (IV) oxide nanoparticles, preparedaccording to the proportions used in Example 8, when tested in thephenol photo-oxidation test has a low photoactivity rate constant of1.8×10³ min⁻¹, (rate of disappearance of phenol) in a phenolphoto-oxidation test as compared to that of a standard titanium (IV)oxide product, Degussa P25, which had a rate constant of 28.5×10³ min⁻¹.

Example 26

This example shows that use of sodium pyrophosphate as base makes anamorphous product.

Example 5 was repeated, except that 13.3 mL titanium (IV) isopropoxidewas added to the solution of 600 mL isopropyl alcohol and 2.0 mL H₂O. Aportion of the prepared high optical transparency hydrous titanium (IV)oxide nanoparticle suspension was evaporated to dryness at roomtemperature. X-ray powder diffraction showed the powder to be amorphous.

Example 27

A suspension of hydrous titanium (IV) oxide nanoparticles, containingabout 0.5 wt. % TiO₂ on a dry basis, was prepared according to theproportions used in Example 8. A powder was obtained from the suspensionby evaporating a portion of the suspension. The powder was sieved to−200 mesh.

Example 28

Example 8 was repeated except that 6.7 mL titanium (IV) isopropoxide wasadded to the solution containing 300 mL isopropyl alcohol and 2 mL H₂O.The high optical transparency suspension, containing about 0.5 wt. %TiO₂ (dry basis) was evaporated to dryness at room temperature torecover a powder. The recovered powder was shown to be amorphous fromX-ray powder diffraction.

Example 29

This example describes the synthesis of a concentrated suspension viavacuum distillation.

A suspension of hydrous titanium (IV) oxide nanoparticles was preparedin a similar manner to that described in Example 28. Component “A” wasprepared by mixing 900 mL isopropyl alcohol with 6.0 mL deionized waterfollowed by the addition of 20 mL titanium (IV) isopropoxide. Component“B” was made by mixing about 1.5 mL concentrated ammonium hydroxide(NH₄OH) with 300 mL deionized water. Component “B” was rapidly added tostirring Component “A”. A highly optically transparent suspension ofhydrous titanium (IV) oxide nanoparticles, resembling a clear colorlesssolution, was formed. To the suspension was added 8.4 mL oftetraethylammonium hydroxide solution (25 wt. % in methanol) withstirring to achieve a weight percent concentration of tetraethylammoniumhydroxide of approximately 0.15 wt. %. The synthesis was repeated threetimes, and the four batches of suspension were used as the sourcematerial that was vacuum distilled at about 30° C. to produce aconcentrated suspension that was fluid and relatively opticallytransparent having a slightly translucent appearance as compared to thesource system. The hydrous titanium (IV) oxide nanoparticleconcentration was about 10.1 wt. %, based on TiO₂. A portion of thisconcentrate was diluted with water to about 0.1 wt. % TiO₂ for PSDdetermination. The median particle size was about 8 nm as measured bydynamic light scattering (see FIG. 5).

Example 30

This example describes the synthesis of a concentrated suspension viavacuum distillation.

A suspension of hydrous titanium (IV) oxide nanoparticles was preparedin a similar manner to that described in Example 28. Component “A” wasprepared by mixing 900 mL isopropyl alcohol with 6.0 mL deionized waterfollowed by the addition of 20 mL titanium (IV) isopropoxide. Component“B” was made by mixing about 1.7 mL concentrated ammonium hydroxide(NH₄OH) with 300 mL deionized water. Component “B” was rapidly added tostirring Component “A”. A highly optically transparent suspension ofhydrous titanium (IV) oxide nanoparticles, resembling a clear colorlesssolution, was formed. To the suspension was added 10 mL oftetra-n-butylammonium hydroxide solution (31 wt. % in methanol) withstirring to achieve a weight percent concentration oftetra-n-butylammonium hydroxide equal to approximately 0.25 wt. %. Thesynthesis was repeated seven times, and the eight batches of suspensionwere used as the source material that was vacuum distilled at about 30°C. to produce a concentrated suspension that was fluid and relativelyoptically transparent having a slightly translucent appearance ascompared to the source system. The hydrous titanium (IV) oxidenanoparticle concentration was about 17.8 wt. %, based on TiO₂.

Example 31

This example describes the direct synthesis of concentrated suspensions.

While stirring with a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 150 mL isopropyl alcohol with 1.0 mL deionized waterfollowed by the addition of 10 mL titanium (IV) isopropoxide. Component“B” was made by mixing 5 mL tetra-n-butylammonium hydroxide solution (31wt. % in methanol) into 50 mL deionized water. Component “B” was rapidlyadded to stirring Component “A”. A highly optically transparentsuspension of hydrous titanium (IV) oxide nanoparticles, resembling aclear colorless solution, was formed after stirring for a few minutes.The molar ratio of H₂O:Ti in the suspension was about 84 to about 1. ThepH of the nanoparticle suspension was ˜8. On a dry TiO₂ basis, thesuspension contained about 1.4 wt. % TiO₂.

The synthesis was repeated with 20 mL titanium (IV) isopropoxide and 10mL tetra-n-butylammonium hydroxide solution (31 wt. % in methanol).Initially, a white, opaque suspension was formed that transformed, overa period of about 20 minutes, into a fluid and highly opticallytransparent suspension of hydrous titanium (IV) oxide nanoparticlesresembling a clear colorless solution. The pH of the nanoparticlesuspension was ˜8. On a dry TiO₂ basis, the suspension contained about2.7 wt. % TiO₂.

Example 32

This example illustrates reaction of titanium (IV) isopropoxide (a.k.a.,Tetraisopropyltitanate, Ti(OCH(CH₃)₂)₄) in isopropyl alcohol containingabout 0.8 wt. % H₂O with a different base, tetraethyl ammoniumhydroxide.

With stirring from a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 100 mL of isopropyl alcohol containing about 0.8 wt.% H₂O, with 4.0 mL titanium (IV) isopropoxide. Component “B” was made bymixing 9 drops (˜0.45 mL) of a 25 wt. % solution of tetraethylammoniumhydroxide in methanol with 33 mL deionized water. Component “A” wasstirred in a Pyrex beaker with a Teflon-coated magnetic stirring bar asComponent “B” was rapidly added to Component “A”. A highly opticallytransparent suspension of hydrous titanium (IV) oxide nanoparticles,resembling a clear colorless solution, was formed. The molar ratio ofH₂O:Ti in the suspension was about 139 to about 1. On a dry TiO₂ basis,the suspension contained about 0.9 wt. % TiO₂.

Example 33

This example demonstrates how turbulence intensity when mixingComponents “A” and “B” can impact agglomeration of the nanoparticles.Component “A” was prepared by first adding enough water to dry isopropylalcohol to attain a water content of approximately 0.8 wt. % and thenstirring in titanium isopropoxide to achieve a titanium isopropoxideconcentration of about 2.6 wt. %. Component “B” was prepared by stirringenough concentrated NH₄OH into water to attain an NH₄OH concentration ofabout 0.16 wt. %. Three different means were used to combine A with B toproduce nanoparticles. The first experiment used a homogenizer with arotor/stator configuration, where the rotational speed of the bladescould be varied from 0 to 10,000 rpm. The shear rate, defined above,ranged from 0 to 13,660 s⁻¹ as shown in Table 1 (a-d). To compare, theshear rate used in Comparative Example C was only about 16 s⁻¹.Components “A” and “B” were fed into the homogenizer at constant flowrates of 120 and 40 mL/min, respectively. Samples of the product of theExample were taken under the same flow conditions for a number ofrotational speeds; the results of the particle size distribution andoptical transparency analysis are reported in Table 1. Example 33b(produced with the rotor spinning at 2000 rpm), produced particles witha median size of 4.6 nm and a volume-based d₉₀ (90^(th) percentile) sizeof 6.9 nm. The optical transparency of the suspension was 98% (see Table2). Without the shear provided by the spinning blade, the reactionproduced very large particles. The transmission spectrum for the productof Example 33b is reported in FIG. 3 denoted by a filled triangle.

The second experiment (e and f in Table 1) used a T-mixer with twoopposing flows to combine Components “A” and “B”. The T-mixer is shownin FIG. 6. The “T” mixer was composed of a ¼ inch (0.630 cm) diametermixing tube 2 having an upper portion 3 with two feed tubes 4 a and 4 bfitted at the upper portion of the mixing tube 2 each feed tube havingan end 6 a and 6 b positioned perpendicular to the ¼ inch diametermixing tube each feed tube for introducing one of the solutions into themixing tube. The feed tubes were positioned within the mixing tube suchthat there was a ⅛ inch (0.318 cm) gap 7 located between the ends ofeach feed tube. In this Example the feed tube diameter for Component “A”was ⅛ inch (0.318 cm) in diameter, the feed tube diameter for Component“B” was 1/16 inch (0.160 cm) in diameter. All diameters refer to theouter diameter of the tube. The flow rate of Component “B” was fixed at8 mL/min, and two flow rate conditions were tried for Component “A”: 20mL/min (a total flow of 28 mL/min) and 32 mL/min (a total flow of 40mL/min). Evidently the mixing achieved under these conditions was not asgood as with the homogenizer, because the product was significantlylarger and less optically transparent.

A third experiment (g in Table 1) used a static mixer to combine A and Bunder higher flow conditions, and the result was a product comparable tothat obtained with the homogenizer.

TABLE 1 Effect of Mixing Conditions Optical Operating Shear d₁₀ d₅₀ d₉₀Transparency Sample Equipment Condition rate (s⁻¹) (nm) (nm) (nm) (%) aHomogenizer rotor 0 ~0 324 2566 >3000 2 rpm b Homogenizer rotor 20002730 2.6 3.7 5.6 98 rpm c Homogenizer rotor 5000 6830 2.8 3.8 5.6 98 rpmd Homogenizer rotor 13,660 3.1 4.1 5.7 97 10,000 rpm e T-mixer flow ratenot 10 12 40 11 28 mL/min known f T-mixer flow rate not 8 10 22 18 40mL/min known g Static mixer flow rate not 4 5 9 98 over 100 known mL/minComp. Stirring bar in stir bar 16 54000 <0.01 Example a beaker 400 rpm C

Stability of suspensions as demonstrated by the absence of a settledlayer of agglomerated particles has been observed after a period of overone year.

Example 33 demonstrates that when the mixing was accomplished withsufficient vigor the desired nanoparticles were formed.

FIGS. 1 a and 1 b show the volume-weighted median particle sizedistribution, determined by dynamic light scattering, of directly-madetitanium (IV) oxide suspensions of several of the Examples. FIG. 1 a isa plot of the weight fraction at size (%) v. size (nm) for the productsof Examples 4, 7, 8, and 14. The particle size distributions are shownto lie in the range of about 2—about 7 nm, with a typical medianparticle size of approximately 5 nm.

FIG. 1 b is a plot of the weight fraction at size (%) v. size (nm) forthe products of Examples 18, 19 and 20. The particle size distributionsare shown to lie in the range of about 1 to about 10 nm with a smallshoulder for the product of Example 18 between about 10 and about 50 nm.

FIG. 2 shows that the absorption spectrum for a 0.1 wt. % suspension ofnanoparticles made by diluting a 3 mL suspension of Example 4 with 9 mLof water (denoted in the figure by filled circles) is nearly the same asthat for a 0.1 wt. % suspension of the plasma-produced anatase titania(denoted by open triangles). The absorbance curves have a slightlydifferent shape due to scattering caused by larger particles (mediansize 30 nm) in the plasma-produced sample. Both the amorphous andcrystalline titanium (IV) oxide particles absorb ultraviolet energy atwavelengths shorter than about 335 nm, whereas in the visible spectrumthe absorption is very low. As predicted by Beer's Law, the absorbanceat a given wavelength is four times greater for a 0.4 wt. % suspensionof the amorphous titanium (IV) oxide of Example 4 (denoted in FIG. 2 byopen circles) than for the 0.1 wt. % suspension. The weight percent wasbased on the weight of dry titanium (IV) oxide that could be obtainedfrom the suspension.

In FIG. 3 the measured transmission coefficient (%) is shown as afunction of wavelength (nm) over the visible part of the electromagneticspectrum for products made in accordance with Comparative Example C andExamples 7, 9, 10, 18 and 33b. The wavelength-dependent sensitivity ofthe human eye (the photopic luminous efficiency function, published bythe International Committee on Illumination (CIE) in 1988) is shown forreference as a dashed line. With the exception of Comparative Example C,these samples were prepared according to the Examples 7, 9, 10, 18 and33b and have not been diluted or otherwise altered in any way.

FIG. 3 shows that even when the sample of the product of ComparativeExample C is diluted to a solids concentration of 0.1 wt. %, based onthe entire weight of the diluted sample, it is almost completely opaqueas shown by its transmission spectrum, denoted by the dashed line taggedwith an “x”. In contrast, the measured transmission spectra of theexamples produced by the present disclosure are seen to have varyingdegrees of optical transparency. For example, Example 9 (using a minimalamount of base) is denoted by the line with the open square. Example 10(made using a larger concentration of base than Example 9), denoted bythe line with the open triangle, transmits significantly more light.Example 18 (denoted by an open circle) shows that the suspensions madewith the present disclosure remain relatively clear even after 2 monthsof storage. Example 7 (denoted with a filled circle) and Example 33b(denoted with a filled triangle) exhibit extraordinarily high opticaltransparency. The optical transparency for each of the spectra shown inFIG. 3 was calculated and the results reported in Table 2.

TABLE 2 Optical Transparency of TiO₂ Samples to Visible Light SampleTransparency to Visible Light Comparative Example C <0.01%    Comparative Example C (diluted 22:1) 0.2%  Example 9  4% Example 10 33%Example 18 (aged 2 months) 66% (after 2 months) Example 7 95% Example33b 98% Pure, ultrafiltered water 100% 

According to Beer's Law, an increase in the solids concentration willhave the same effect as increasing the optical path length: the opticaltransparency will decrease. Therefore before any direct comparisonbetween two samples can be made, one must ensure that their particleconcentrations are equal. In the Examples discussed above, theconcentration was about 0.5 wt. %. The undiluted Comparative Example Cas about 2.2 wt. %, but even when diluted to 0.1 wt. %, the ComparativeExample was still opaque.

The reason for the varying amount of optical transparency observed invarious nano titanium (IV) oxide samples is that the particle sizedistribution (PSD) or refractive index (which is determined by thecrystallinity) was not the same. FIG. 4, shows the data obtained bypreparing a number of 0.1 wt. % titanium (IV) oxide suspensions andmeasuring both median particle size and optical transparency (% T). InFIG. 4, triangles denote suspensions of amorphous titanium (IV) oxidemade in accordance with Examples 9, 18, and 33 and diluted to 0.1 wt. %;circles denote suspensions of rutile titanium (IV) oxide made via thecombustion of TiCl₄, and the square denotes the suspension made inaccordance with Comparative Example C. It is evident that the reason forthe opacity of the suspension produced in Comparative Example C is thatits median particle size is 10,000 times larger than that of thesuspensions of the Examples.

The optical transparency of a suspension is determined by the complexrefractive index and size distribution of the particles and by therefractive index of the liquid. The scattering of light by particles ina suspension is a well-studied phenomenon, the theoretical framework ofwhich was completely described by Gustav Mie [Annalen der Physik25:377-445 (1908)] nearly 100 years ago.

The solid line in FIG. 4 was the result of calculations made based onMie scattering theory, where a log-normal distribution with a geometricstandard deviation of 1.1 was assumed for the PSD and the complex indexof refraction for amorphous titanium (IV) oxide was provided by B.Karunagaran et al., Cryst. Res. Technol. 38:773-778 (2003). Thecalculation assumed an optical path length of 1 cm and a particleconcentration of 0.1 wt. %. Both theory and empirical data show thatsuspensions of particles with diameters greater than about 100 nm willbe opaque, and that very optically transparent suspensions (based onvisual appearance, estimated % T>90%) are obtained only when the medianparticle size is smaller than about 10 nm. Suspensions with wideparticle size distributions (those with larger geometric standarddeviations) will be less optically transparent than predicted here dueto increased scattering of light caused by the larger amount of coarseparticles.

It is noted that “white turbid” suspensions appear white because theparticles in them scatter visible light completely. Since scatteredlight is not transmitted in the direction of the incident light rays,the transmission coefficient goes to zero over a relatively short pathlength in such suspensions. The theoretical and empirical results shownin FIG. 4 show that such suspensions have median particle sizes thanexceed 100 nm.

Both theory and empirical data reported in FIG. 4 showed thatsuspensions of particles with diameters greater than about 100 nm are ofnecessarily opaque due to increased scattering of light by the largerparticles, and that very optically transparent suspensions (% T>90%) canbe obtained only when the median particle size is smaller than about 10nm. Of course, suspensions with wide particle size distributions (thosewith larger geometric standard deviations) are expected to be lessoptically transparent than predicted here due to increased scattering oflight caused by the larger amount of coarse particles.

Example 34

In this Example the titanium (IV) oxide nanoparticles were surfacetreated with a thin silica coating to reduce further the inherently lowphotoactivity.

A solution was made by stirring 1 mL of TEOS (Si(OCH₂CH₃)₄) into 19 mLof isopropyl alcohol; this TEOS solution was loaded into a syringe pump.The pump was set to deliver the TEOS solution at a rate of 1.5 mL/hrinto a stirred beaker containing 100 mL of the suspension made inExample 5. The TEOS solution was fed into the suspension while stirringwith a magnetic stir bar continuously for 4 hours. Samples (about 2 mLeach) were taken at one-hour intervals for particle size measurement.

After 2 hours, about 10% of the particle volume was agglomerated to asize of about 16 nm, but this agglomeration decreased with time; at 3hours this agglomeration contained only a few percent of the totalparticle volume, and after 4 hours the agglomeration disappearedcompletely. After a total of 4 hours, the syringe pump was turned offand the beaker was set aside (without stirring) overnight. The nextmorning some of the previously optically transparent suspension hadagglomerated and formed a less optically transparent layer at the bottomof the beaker, but the top half of the suspension was still opticallytransparent. The larger particle size in the bottom half of the beakeris evidence that some agglomeration or flocculation had occurredovernight.

The particle size of the suspension were reported in Table 3 as afunction of time and the total TEOS addition.

TABLE 3 Time Total TEOS Median (hours) (mL) size (nm) 0 0 5.7 1 1.5 5.72 3.0 6.4 3 4.5 7.2 4 6.0 7.2 about 20 6.0 4.9 (top half) about 20 6.010.0 (bottom half)

The suspension was decanted into a separate container and dried to forma powder.

Example 35

In this Example a suspension of amorphous hydrous titanium (IV) oxidenanoparticles were produced in a continuous or semi-continuous process.

Using the same proportions of Example 33 to produce sufficientquantities of Component “A” and Component “B”, the two components werecombined using three different continuous mixing means as shown in Table6. All three mixing means were observed to be capable to producing thesuspension of this disclosure.

The homogenizer had a rotor/stator configuration arranged so that thetwo components were fed on opposite sides of the stator plate, thusensuring maximum mixing.

The T-mixer was made from commercial tubing and tube fittings and asshown in FIG. 6. The feed tube for Component “A” was made of 3.2 mmtubing, the feed tube for Component “B” was made of 1.6 mm tubing, andthe end of each feed tube was separated by a 3.2 mm gap located in a 6.4mm tubing for the mixing tube to form a T-mixer. The cross-sectionalareas of the tubes were chosen so that the volumetric flow rate requiredto mix the two components in the correct ratio would yield approximatelyequal velocities of the two jets. The tube diameters refer to the outerdiameter of the tube.

The static mixer was formed from a commercial static in-line mixer, withthe Component “B” feed tube inserted into the middle of the mixer. Thisarrangement was used to produce about 10 liters of suspension at a rateof about a liter per minute. This process can be scaled to much largerproduction rates.

Example 36

This Example demonstrates the production of a dry flowable powder fromthe suspension produced by the static mixer in Example 35. This powdercan be re-dispersed in a liquid if desired.

Feed suspension produced by the static mixer in Example 35 was distilledto a concentration of about 1.5% (on a dry TiO₂ basis). The concentratedsuspension was fed through an atomizing nozzle into a 1-meter diameterspray drying chamber. Compressed nitrogen was supplied to the annulargap of the nozzle at a pressure of 30 psig. Drying nitrogen (heated to120-130° C.) was supplied to the chamber at a rate of 60-80 kg/hr. Thedryer exhausted to a small bad filter containing 9 filter elements ofTeflon® coated polyester with a combined filtration area of about 0.8m². Due to the fine size of the powder, the filter bags were notback-pulsed during the drying step. In order to separate the powder fromthe filters, the bags were back-pulsed at the conclusion of the dryingrun, without drying gas or feed flowing to the dryer. Over 130 g ofpowder was produced; this powder was extremely light, easily fluidized,and easily entrained in gas flow. The powder, once fluidized (in air),was slow to de-aerate. The bulk density of de-aerated powder was onlyabout 320 kg/m³.

The particles in this powder were observed with SEM to be agglomeratesof titanium (IV) oxide nanoparticles. Laser diffraction measurements ofthe particle size distribution of the dry powder showed the d₅₀ to beabout 2 micrometers, although a substantial fraction (about 20% of thevolume) was less than 1 micrometer. When re-dispersed in water withsonication for 1 minute, the d₅₀ (median size) of the suspension wasmeasured with dynamic light scattering to be about 90 nm.

Example 37

This example illustrates that reaction of titanium (IV) isopropoxide(a.k.a., Tetraisopropyltitanate, Ti(OCH(CH₃)₂)₄) in isopropyl alcoholcontaining about 1.6 wt. % H₂O with a base gives the nano-sizedparticles of hydrous titanium (IV) oxide but transforms within 10seconds into an opaque slurry. The water to titanium molar ratio of themixture was about 286. While stirring with a Teflon-coated magnetic stirbar, Component “A” was prepared by mixing 300 mL isopropyl alcohol with4.0 mL deionized water followed by the addition of 6 mL titanium (IV)isopropoxide. Component “B” was made by dissolving 0.72 gtetrasodiumpyrophosphate decahydrate (TSPP) in 100 mL deionized water.Component “A” was stirred in a 600 mL beaker with a Teflon-coatedmagnetic stirring bar as Component “B” was rapidly added to Component“A”. A highly optically transparent suspension resembling a clearcolorless solution was formed that transformed into an opaque whiteslurry within 10 seconds.

Example 38

This example illustrates the result of reaction of titanium (IV)isopropoxide (Tetraisopropyltitanate, Ti(OCH(CH₃)₂)₄) in isopropylalcohol containing about 0.8 wt. % H₂O with a sodium hydroxide as thebase.

With stirring from a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 150 mL isopropyl alcohol with 1 mL deionized waterfollowed by the addition of 3 mL titanium (IV) isopropoxide. Component“B” was made by mixing one drop (˜0.04 mL) 10 M NaOH with 50 mLdeionized water. The pH of Component “B” was about 11. Component “A” wasstirred in a 400 mL beaker with a Teflon-coated magnetic stirring bar asComponent “B” was rapidly added to Component “A”. A highly opticallytransparent suspension resembling a clear colorless solution of hydroustitanium oxide nanoparticles was formed. On a dry TiO₂ basis, thesuspension contained about 0.5 wt. % TiO₂.

Example 39

This example illustrates the result of slow mixing of titanium (IV)isopropoxide (Tetraisopropyltitanate, Ti(OCH(CH₃)₂)₄) in isopropylalcohol containing about 0.8 wt. % H₂O with NH₄OH as the base.

With stirring from a Teflon-coated magnetic stir bar, Component “A” wasprepared by mixing 150 mL isopropyl alcohol with 1 mL deionized waterfollowed by the addition of 3 mL titanium (IV) isopropoxide. Component“B” was made by mixing four drops (˜0.12 mL) concentrated NH₄OH with 50mL deionized water. Component “A” was stirred in a 400 mL beaker with aTeflon-coated magnetic stirring bar as Component “B” was added in asteady stream over a period of about 30 seconds. An opticallytranslucent suspension of hydrous titanium (IV) oxide nanoparticles wasformed. The optically translucent suspension produced no settled solidsafter sitting for six hours.

The description of illustrative and preferred embodiments of the presentinvention is not intended to limit the scope of the invention. Variousmodifications, alternative constructions and equivalents may be employedwithout departing from the true spirit and scope of the appended claims.

1. A process for making a nanoparticle suspension of titanium (IV) oxideparticles, comprising the following steps: 1) mixing with a vigorequivalent to a shear rate of greater than 100 s⁻¹ (a) and (b) wherein(a) is a volume of a first component comprising an alcohol in an amountof more than 50 wt. % based on the weight of the first component, atitanium alkoxide in an amount ranging from 0.001 wt. % to 6 wt. % basedon the weight of the first component, and a titanium alkoxide activatorin an amount ranging from 0.2 wt. % to 1.6 wt. % based on the weight ofthe first component, the titanium alkoxide activator being selected fromthe group consisting of water and a first aqueous base selected from thegroup consisting of ammonium hydroxide, sodium hydroxide, sodiumpyrophosphate, potassium pyrophosphate, sodium citrate, diammoniumhydrogen phosphate and mixtures thereof, and (b) is a volume of a secondcomponent selected from the group consisting of water and a secondaqueous base selected from the group consisting of ammonium hydroxide,sodium hydroxide, sodium pyrophosphate, potassium pyrophosphate, sodiumcitrate, diammonium hydrogen phosphate and mixtures thereof, and atleast one of the first component or the second component having a basetherein, 2) rapidly forming a mixture comprising a suspension oftitanium (IV) oxide nanoparticles having a volume-weighted medianparticle size measured via dynamic light scattering below approximately100 nm, the mixture having a water to titanium molar ratio ranging fromabout 40 to about 1 to about 5000 to about
 1. 2. The process of claim 1in which the amount of water of the first component is provided by thealcohol wherein the alcohol comprises water of hydration.
 3. The processof claim 1 in which amount of the titanium alkoxide activator is about0.8 wt. % based on the entire weight of the first component.
 4. Theprocess of claim 1 in which the titanium alkoxide has the chemicalstructure:Ti(OR₁)(OR₂)(OR₃)(OR₄) wherein R₁ to R₄ are the same or different alkylgroups of 1 to about 10 carbon atoms.
 5. The process of claim 1 in whichthe titanium alkoxide is selected from the group consisting of titanium(IV) isopropoxide, titanium (IV) n-butoxide, titanium (IV) methoxide,titanium (IV) ethoxide, and titanium (IV) n-propoxide and mixturesthereof.
 6. The process of claim 1 in which the base of the firstaqueous base is in a concentration up to about 0.2 M in the firstcomponent.
 7. The process of claim 1 in which the base of the secondaqueous base is in a concentration ranging from about 0.001 M to about0.030 M in the second component.
 8. The process of claim 1 in which thebase of the second aqueous base is ammonium hydroxide in a concentrationranging from about 0.006 M to about 14.8 M in the second component. 9.The process of claim 1 in which the alcohol is selected from the groupconsisting of methanol, ethanol, n-propanol, isopropanol, ethyleneglycol, and butanediol and mixtures thereof.
 10. The process of claim 1in which the titanium alkoxide activator consists essentially of waterand is substantially free of base.
 11. The process of claim 1 in whichthe second component is substantially free of base.
 12. The process ofclaim 1 in which the mixing is accomplished in a homogenizer wherein therotor speed is greater than about 500 rpm.
 13. The process of claim 1further comprising drying the suspension of titanium (IV) oxidenanoparticles to form a powder having the chemical composition:TiO_(2-x)(OH)_(2x) .n(H₂O) wherein x ranges from 0-2 and n is no greaterthan about
 2. 14. The process of claim 1 in which the suspension oftitanium (IV) oxide nanoparticles comprises the titanium (IV) oxideparticles in a liquid medium and the process further comprisesconcentrating the suspension by removing at least a portion of theliquid medium.
 15. The process of claim 1 further comprising mixing thesuspension of titanium (IV) oxide nanoparticles with a polymer.
 16. Theprocess of claim 1 in which the suspension of titanium (IV) oxidenanoparticles has an optical transparency greater than about 5% over a 1cm path length.
 17. The process of claim 1 in which the suspension oftitanium (IV) oxide nanoparticles has an optical transparency greaterthan about 90% over a 1 cm path length.
 18. The process of claim 1 inwhich the second component is free of alcohol.
 19. The process of claim1 in which the mixture has a pH greater than or equal to about
 6. 20.The process of claim 1 wherein the process takes place at roomtemperature.